Pelletizing process

ABSTRACT

WETTED POWDERS ARE PELLETIZED IN AN ELONGATED PELLETIZING ZONE BY MEANS OF INTERDIGITABLE AGITATING MEMBERS WHICH PROJECT RADIALLY OUTWARDS FROM TWO ROTATABLE SHAFTS WHICH EXTEND AXIALLY THROUGH THE ZONE. A CURVED, LONGITUDINALLY EXTENDING WALL OF A CONDUIT WHICH BOUNDS THE PELLETIZING ZONE IS PROXIMAL TO THE TIP-ENDS OF THE AGITATING MEMBERS WHICH ARE NOT INTERDIGITATING DURING ROTATION OF THE SHAFTS. DURING PELLETIZATION THE WETTED POWDER ADVANCES AXIALLY THROUGH THE CONDUIT, FIRST BEING SWIRLED AGAINST THE CURVED CONDUIT WALL, AND THEN TOWARD THE CENTER LINE OF THE CONDUIT WHERE IT IS TRANSECTED BY INTERDIGITATING MEMBERS. THE MEMBERS WHICH ARE NOT INTERDIGITATING MOVE THROUGH THE WETTED POWDER AGAINST THE WALL. THROUGH THE RECIRCULATION OF WETTED POWDER FROM THE WALL, THROUGH THE INTERDIGITATING MEMBERS, AND BACK TO THE WALL AS THE POWDER ADVANCE AXIALLY THROUGH THE PELLETIZING ZONE.

Filed June 25, 1971 F, N. HARE PELLETIZ lNG PROCESS 5 Sheets-Sheet 2 F. N. HARE PELLETIZING PROCESS 3 Sheets-Sheet 3 Filed June 25, 97l

UnitedStates Patent O U.S. Cl. 264-117 6 Claims ABSTRACT OF THE DISCLOSURE Wetted powders are pelletized in an elongated pelletizing zone by means of interdigitable agitating members which project radially outwards from two rotatable shafts which extend axially through the zone. A curved, longitudinally extending wall of a conduit which bounds the pelletizing zone is proximal to the tip-ends of the agitatmg members which are not interdigitating during rotation of the shafts. During pelletization the wetted powder advances axially through the conduit, first being swirled against the curved conduit wall, and then toward the center line of the conduit where it is transected by interdigitating members. The members which are not interdigitating move through the wetted powder against the wall. There is recirculation of wetted powder from the wall, through the interdigitating members, and back to the wall as the powder advances axially through the pelletizing zone.

BACKGROUND OF THE INVENTION The present invention pertains to an improved process for pelletizing powders by wetting the powder with a liquid pelletizing agent and agitating the resulting wetted powder mass to form the pellets. The present invention resides in the general area of agitating wetted powder to form pellets by means of a pelletizer having an elongated pelletizing zone, a rotatable shaft which extends through the zone, and a multiplicity of agitating members fixed at spaced intervals along the length of the shaft and which extend radially outward therehom. By rotation of the shaft, a wetted powder is thus agitated and pelletized while also being advanced from the powder inlet of the pelletizing zone to the pellet outlet thereof. Generally, though not necessarily, the resulting pellets are dried after formation to remove the pelletizing liquid.

In accordance with prior methods, only one shaft having agitating members aixed thereto is usually employed for pelletizing a wetted powder. By rapid rotation of the shaft, the wetted powder mass is not only moved axially through the peiletizing zone but is also slung outward against the curved wall of the `zone where it forms an annular mass. The tips of the agitating members move through this annular mass, thus cutting through it and thereby agitating and forming pellets of the powder particles in the mass, and also imparting some degree of rotary motion thereto, i.e. there is a tendency for the annular mass to spin in conjunction with the agitating members, and were it not for the friction provided by the pelletizer wall, the mass would spin at the same speed as the shaft and thus eliminate any agitation of the mass caused by transversal movement of the agitators through the same. Therefore, although effective .for the formation of pellets, there is nonetheless a limit to the amount of pelletizing work which can be transferred into the wetted powder mass by means of the method just described.

In order to increase the amount of work which can be effectively utilized to advantage in pelletizing wetted powders, coinventors Glaxner and Kilpatrick provided a method and an apparatus as described in U.S. Pats. 2,828,191 and 2,861,294 respectively. As shown therein, the second pelletizing chamber is provided with three parallel shafts having agitating members. The agitating 3,75l,5543 Patented Aug. 7, 1973 members of each shaft are arranged to move into and out of interdigitating relationship with agitating members of an adjacent shaft upon rotation of the shafts. Agitation of the wet pellets is thus accomplished by interdigitation of the members; i.e. the mass of pellets is very .forcefully cut by closely spaced members which move in opposite directions upon counterwise rotation of the shafts.

However, in the method disclosed by Glaxner and Kilpatrick in the aforementioned patents, the movement of the powder mass from inlet to outlet of chamber is transverse, rather than axial, with respect to extension of the shafts. ln addition, the wall of chamber does not conform to the circular path followed by the agitating members when the shafts are rotated and is thus not proximal to the tip-ends of the members. As a consequence of such an arrangement, there is little or no drag of the wetted powder against the wall of chamber, and this aspect of developing pelletizing work is, therefore, essentially nonexistent within the system. It will also be noted that there is no provision for concurrent mixing of powder and liquid and the formation of pellets in chamber, i.e. these steps are accomplished beforehand in a first chamber.

This particular method of Glaxner and Kilpatrick is, therefore, somewhat elaborate and not altogether satisfactory for developing maximum work. It will be appreciated that pelletization rate, density, and the amount of pelletizing liquid required are a function of the amount of work delivered to the wetted powder mass during agitation, i.e. as the work is increased the pellet .formation rate and density increase and the required amount of pelletizing liquid decreases.

SUMMARY OF THE INVENTION It is therefore an object of this invention to provide a process of increasing the transfer of work into a mass of powder during wet pelletization of the powder.

Another object is to provide a process of increasing the formation rate of pellets of a powder.

Still another object is to provide a process to produce higher density pellets of a powder.

Yet another object is to provide a process to reduce the amount of pelletizing liquid required to wet pelletize a powder.

Even other objects and advantages of the invention will become apparent from the following description and the appended claims.

In accordance with the present invention, a wetted mass of powder is pelletized by agitation in an elongated agitating zone having two axially extending rotatable shafts therein, each of the shafts being equipped with elongated agitating members which interdigitate within a region of the pelletizing zone toward the center line thereof. The pelletizing zone has a curved, longitudinally extending periphery which is proximal to the tip-ends of agitating members which are not interdigitated.

By rapidly rotating both shafts, the wetted powder mass is caused to swirl around the peripheral region of the pelletizing zone, and agitating members which are not interdigitated are moved through the wetted powder in this peripheral region. .After having moved around the curving periphery of the pelletizing zone, the wetted powder is discharged from the peripheral region and is passed into the more central region of the zone wherein the agitating members are interdigitating. The wetted powder is then discharged from this central region of the pelletizing zone and is returned to the peripheral region. These steps are repeated as the wetted powder advances axially through the pelletizing zone.

More effective pelletization thus results from coupling the work available from the interdigitating agitating members with that which results from moving agitating members which are not interdigitated through a resisting mass of wetted powder which lies against the wall of the pelletizing zone.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a top view, partly in section, of a pelletizing apparatus, for forming pellets in accordance with the present invention.

FIG. 2 is a sectional view taken along the line 2-2 of FIG. 1 and show agitating members of each shaft in fllfinterdigitating relationship with members of the other s a t.

FIG. 3 is the same as FIG. 2 except for showing the agitating members of each shaft just after they have moved out of interdigitating relationship, this being caused by rotating each shaft through 45 degrees from the positions shown in FIG. 2.

FIG. 4 is a side view, taken along line 4 4 of FIG. l.

FIG. 5 is a side view of the pelletizer of FIG. 1, but equipped with an access panel, or door, in the wall of the pelletizer conduit, the door being shown in the closed position.

FIG. 6 is an end view from the left of the pelletizer of FIG. 5 showing the access door in an open position.

DETAILED DESCRIPTION OF THE INVENTION The pelletizer comprises an elongated pelletizing zone, generally represented at 1, which is bounded by a pelletizing conduit 2. having the cross-sectional configuration of two overlapping circles having the overextending segment of each circle removed, e.g. FIG. 2 or 3, and thus providing communication between the two partially cylindrical bilateral halves of the pelletizing zone over its full length. Each side of the pelletizing conduit, i.e. each partial cylinder, is tted with a rotatable shaft, 3a and 3b. Each shaft extends axially through the pelletizing zone along the axis of the partial cylinders. The shafts are supported by bearings 4 mounted on end-closure plates 5 for the pelletizer conduit. Each shaft is provided with a multiplicity of elongated agitating members 6a and 6b, hereinafter referred to as pins, which are xed at spaced intervals along the length of the shaft and which extend radially outward therefrom. The lateral spacing of the parallel shafts and the length of the pins is such that the pins overlap each other upon rotation of the shafts. However, the longitudinal spacing of the pins along one shaft is staggered with respect to the spacing on the other, so that the pins do not collide upon rotation of the shafts but rather clear each other and thus effect interdigitation of the pins of one shaft with the pins of the other.

Each shaft is equipped with a variable-speed drive unit, generally represented at 7a and 7b consisting of an electric motor 8 and a speed reducer 9.

The pelletizing zone has an inlet 10 toward one end for the introduction of powder to be pelletized and an outlet 11 toward the other end for the removal of pellets.

A liquid supply conduit 12 extends through the conduit 2 for introducing a liquid pelletizing medium into the pelletizing zone. Two such conduits can be employed, as shown in FIG. 6, for introducing liquid into each partial cylinder of the pelletizing conduit. The method and means described in U.S. Pat. 3,535,412 is preferred for injecting the liquid longitudinally into the pelletizing zone and mixing it with the powder therein.

Although not essential, the pelletizer can he provided with a panel, or door, which provides ready access to the pelletizing zone for inspection, cleaning, and replacement or repair of the pins. FIGS. 5 and 6 show the pelletizer equipped with a door 13, which is in fact a removable section of the conduit wall 2, but equipped with anges 14a and 14b so that it can be provided with a seal and fastened in place. For obtaining access to the pelletizing zone, the door is unfastened and is then removed using handles 15. More conveniently, however, the door can be provided with a hinge 16 so that the pelletizer can be more easily opened and closed.

In operation, powder and a liquid pelletizing medium are fed into the pelletizer at predetermined rates through the powder inlet 10 and the liquid conduit 12, respectively. The proportion of powder-to-liquid feed is such that a pelletizing amount of liquid is supplied, i.e. an amount which results in the formation of pasty wet pellets rather than dusting out as occurs from use of insuicient liquid or mudding out which results from using an excess of liquid. The necessary proportions are usually well known to those skilled in the art of pelletizing any particular powder with a given liquid, but can be easily determined in any case by means of a few simple experiments. Where preferred, the powder and pelletizing liquid can be combined prior to introduction into the pelletizer, but mixing of the two can usually be accomplished very quickly and eiciently in the pelletizer, thus oblivating a need for premixing.

Within the pelletizing zone, the wetted powder mass is vigorously agitated by rapid rotation of the shafts 3a and 3b, which causes the pins 6a and 6b to move through the mass. Rotary motion of the shafts gradually advances the wetted powder axially through the pelletizing zone toward the outlet 11, and also rapidly swirls the powder so that it is slung out against and is moved around the curved wall of the conduit 2. However, since the curved extension of the wall is not fully circular, but terminates toward the center of the pelletizing zone, the wetted powder leaves the wall and flies into the central portion of the pelletizing zone between the shafts, and is therein intensely ailed by the interdigitating pins 6a and 6b. There are thus two portions of the wetted powder mass being concurrently subjected to dii'lerent types of agitation Which elTects the pelletization. Referring to FIGS. 2 and 3, one portion, the swirling portion against the wall, lies in section 1a of the pelletizing zone against the curved wall of the conduit 2. The other portion lies toward the center line of the pelletization zone, section 1b.

It will be noted that the curved, longitudinally extending peripheral wall of the conduit 2 closely conforms to the circular path of rotation of the tip-ends of pins 6a and 6b which are not interdigitated. Any of members 6a and 6b which are out of interdigitating relationship during rotation of the shafts are thus moving through that portion of the mass against the wall of conduit 2. As this portion leaves the wall and moves into the central section 1b of the pelletizing zone, it is subjected to the action of pins moving into and out of interdigitation and thereafter moves back to the wall. Accordingly, the mass of wetted powder being agitated and pelletized is repeatedly circulated from the wall of the pelletizing conduit to the interdigitating members and then back to the wall as the mass also advances axially toward the outlet of the pelletizing zone.

It will thus be appreciated that two different agitating actions are carried out concurrently on the same mass of powder, each of which is very intense and adjunctive to the other in the forming of pellets. By virtue of the fact that the conduit wall is proximal t0 the tip-ends of all pins which are not in an interdigitating relationship, none of the work available from their motion is wasted, i.e. the noninterdigitating pins not only move the wetted powder over the pelletizer wall-which in itself causes considerable agitation and tumbling of the particlesbut since the pins are moving relatively faster than the wetted powder, they also cut through it and cause additional agitation and tumbling. Further in addition to this, of course, there is the agitation and tumbling which is effected in the central portion of the pelletizing zone which results from interdigitation of the pins. Intense tumbling results from the close passage of the pins in their movement by each other while the powder is trapped between them. This action is most intense when the Shafts are rotated in the same direction since the pins move in relatively opposite directions during interdigitation, but excellent results can also be obtained when the shafts are rotated in opposite directions and the pins move in the same relative direction.

It will be appreciated that various combinations of shaft speed and rotational direction can be established for varying the conditions of agitation to obtain the most desired results. As previously indicated, the shaft rotation can be the same or opposite, and the shaft speeds can be the same or dilerent to the extent desired. Where preferred, the shafts can be operated using one motor and a gear system which establishes the sought direction of shaft rotation and speed. More convenient, however, is the arrangement described with reference to FIGS. 1-4 wherein independent, variable speed drive units are employed for each shaft. To additional advantage, one or both of these drive units can be reversible for changing the direction of shaft rotation.

The minimum speed at which the shafts should be turned is widely Variable depending upon what particular results desired with respect to mean pellet size, pellet size distribution, production rate, and the like. Generally, speeds which provide a pin tip velocity of at least about feet per second can be employed to advantage, and preferred speeds are generally within the range of about to about 50 feet per second.

It will be understood that although the present invention is being described and claimed with reference to pelletizers having two shafts equipped with interdigitating pins, the invention is not thus limited, i.e. three or more shafts arranged to effect the inventive features described herein must also be regarded as embodiments of the present invention.

The pelletizer described herein has been utilized to great advantage in producing pellets of a powder at a greatly increased rate, which are denser after drying, and at a somewhat reduced requirement of a liquid pelletizing medium. It is felt that these benefits are provided by the ability to transfer substantially larger amounts of pelletizing work into the wetted powder mass during the pelletizing process.

'I'he invention will now be further described with reference to specific examples wherein carbon black was the powder pelletized, but it will be understood that the invention is not limited to the pelletization of carbon black.

EXAMPLE 1 A carbon black having the following properties was pelletized in a pelletizer as shown in U.S. 3,535,412 having a single agitating shaft with radially extending pins. The pins had a length of 71/2 inches. The inside diameter of the cylindrical pelletizing conduit was 8 inches and its length was 36 inches.

Carbon black properties lrface area (electron microscope), m.2/gm. 110 Dibutylphthalate absorption, cc./ 100 gms. 200 Iodine absorption number 290 Transmission, percent 100 The carbon black was fed to the pelletizer at the rate of 90 pounds per hour while water was being introduced at the rate of 165 pounds per hour. Shaft speed of the pelletizer was 950 r.p.m. The resulting wet pellets were then dried to a moisture level of 1.2 percent to provide dry pellets of the carbon black.

In a comparison experiment, this same carbon black was pelletized in accordance with the present invention. The pelletizer was equipped with two shafts having agitating members 7%/2 inches long. Each shaft extended through a partial cylinder having an inside diameter of eight inches and a length of 32 inches. Carbon black was fed to this twin shaft pelletizer at the rate of 90 pounds per hour while water was introduced at the rate of 135 pounds per hour. Both shafts were turned at 900' rpm. and were rotated in the same direction. The resulting pellets were then dried as before. Properties of the dried pellets produced in the two experiments was as follows:

Pellet size, wt.' percent Pour 10 18 35 60 120 density,

mesh mesh mesh mesh mesh lbs/tt! Prior art.-. 0. 1 41. 2 42. 2 13. 7 l. 9 14. 4 Invention- 0. 1 29. 1 60. 1 B. 9 1. 1 1G. 8

EXAMPLE II In a third experiment, the second experiment of Example II was repeated, but the speed of the shafts was i11- creased to 1150 r.p.m. and the carbon black feed rate was raised to 220 pounds per hour while the proportion of water was maintained the same, i.e. 330 pounds per hour. The resulting Wet pellets were dried as before, and the dry pellets had about the same size distribution as in the second experiment of Example I. However, the density of the pellets produced in the present case was 20.2 pounds per cubic foot, whereas the highest density which could be achieved with the prior art process having a single agitator shaft was 15 pounds per cubic foot. Besides raising pellet density and lowering water requirement it should also be noted that the size distribution of the pellets produced in the present process was less pronounced and that the majority were in a most desirable range of 18-35 mesh.

The present invention has been described with reference to particular apparatus, process conditions, material, and the like. It will nonetheless be understood that even other embodiments will become apparent which are Within the spirit and scope of the invention defined in the appended claims.

Therefore, what is claimed is:

1. A method for wet pelletizing carbon black powder with a pelletizing liquid wherein the transfer of work into the resulting wetted powder mass is increased and the amount of pelletizing liquid is reduced below that required by conventional pelletizing yet wherein the weight proportion of pelletizing liquid to the carbon black is suliicient for the formation of wet pellets comprising:

(a) supplying a carbon black powder and a pelletizing liquid to a pelletizer having an elongated pelletizing zone with a carbon black inlet at one end and a wet pellet outlet at the other end, two laterally spaced apart rotatable shaft axially extending through said zone, each shaft having radially extending agitating members which upon shaft rotation interdigitate in a central portion of said pelletizing zone, said zone having a curved longitudinal periphery proximal the tips of said agitating members which are out of interdigitating relationship,

(b) rotating said shafts at a tip velocity of said agitating members of at least about l5 feet per second,

(c) swirling said wetted powder around said longitudinal periphery of the pelletizing zone,

(d) moving noninterdigitating agitating members through said wetted powder swirling around said longitudinal periphery,

(e) discharging the said wetted powder from said longitudinal periphery and into said central portion for agitation therein by interdigitation of said agitating members and returning said wetted powder to said longitudinal periphery, and

(f) repeatedly circulating said wetted powder from said longitudinal periphery to said central portion while axially advancing and pelletizing said wetted carbon black from said inlet to said outlet.

2. The method of claim 1 wherein the pelletizing liquid is selected from the group consisting of water and an aqueous solution of a pellet binder.

3. The method of claim 1 wherein the shafts are rotated at a speed which provides a tip velocity to the agitating members which is within the range of about 20 to about 10 50 feet per second.

4. The method 'of claim 1 wherein the weight proportion of pelletizing liquid to carbon black in the wetted powder mass is within the range of about 35/ 65 to about 65/ 35.

5. The method of claim 1in which at least a portion of the pelletizing liquid is introduced into the pelletizing zone separately of the black.

8 6. The method of claim 5 in which the carbon black powder is introduced into the pelletizing zone as substantially dry powder, and mixing of the carbon black and the pelletizing liquid is effected Within the pelletizing zone.

References Cited UNITED STATS PATENTS 3,049,750 8/ 1962 Austin 23-314 2,213,056 8/1940 Skoog et al. 23-314 `FOREIGN PATENTS 282,647 10/ 1965 Australia 425-222 DONALD J. ARNOLD, Primary Examiner 15 I. R. HALL, Assistant Examiner 

